To meet the continued fast growing demand of mobile data, the wireless industry needs solutions that can provide very high data rates in a coverage area to multiple users simultaneously including at cell edges at reasonable cost. Currently, the wireless telecom industry is focused on dense deployment of small cells, the so called ultra-dense networks, to increase spatial re-use of wireless spectrum as the solution for meeting the growing mobile data demand. Dense deployment of small cells requires a large number of backhauls and creates highly complex inter-cell interference. One solution to the interference problem is to require careful Radio Frequency (RF) measurement and planning and inter-cell coordination, which significantly increases the cost of deployment and reduces the spectral efficiency. Another solution is the Self-Organizing Network (SON) technology, which senses the RF environments, configures the small cells accordingly through interference and Tx management, coordinated transmission and handover. SON reduces the need for careful RF measurement and planning at the cost of increased management overhead and reduced spectral efficiency. The backhaul network to support a large number of small cells is expensive to be laid out.
Another method for increasing spatial re-use of wireless spectrum is MIMO, especially Multi-User MIMO (MU-MIMO). In a wireless communication system, a wireless node with multiple antennas, a Base Station (BS) or a User Equipment (UE), can use beamforming in downlink (DL) or uplink (UL) to increase the Signal-to-Noise Ratio (SNR) or Signal-to-Interference-plus-Noise Ratio (SINR), hence the data rate, of the links with other wireless nodes. MU-MIMO can beamform to multiple UEs simultaneously in a frequency and time block, e.g., a Resource Block (RB), i.e., using spatial multiplexing to provide capacity growth without the need of increasing the bandwidth. In a large-scale MIMO or massive MIMO system, a BS may be equipped with many tens to hundreds of antennas. In order for the BS to beamform to multiple UEs using the plural of antennas, the BS needs to know the DL channels to the UEs sufficiently accurately, e.g., the DL Channel State Information (CSI) of each UE. However, it is not efficient to obtain the DL CSI directly by sending reference pilots in the downlink because of two reasons: (1). The large number of antennas on the BS would cause large system overhead for reference signals in the downlink; (2). Dozens of bits are needed to quantize the CSI accurately, which causes overload of the feedback channel in the UL. Fortunately, the reciprocal property of an over the air wireless channel, such as in a Time-Division Duplexing (TDD) system or in a Frequency-Division Duplexing (FDD) system using switching to create channel reciprocity as described in our PCT application PCT/US14/71752 filed on Dec. 20, 2014, claiming the benefit of provisional patent application 61/919,032 filed on Dec. 20, 2013, can be employed to reduce the channel estimation overhead. In such a system, a UE sends a pilot signal, e.g., Sounding Reference Signal (SRS), which is received by all the antennas on the BS in the UL. The BS estimates the UL CSI through the received pilot signal and uses it to estimate the DL CSI based on channel reciprocity.
Although a MIMO BS with a large number of antennas can extend its DL coverage range through beamforming, the SINR of UEs can decay quickly as the distance between the BS and a UE increases, because UEs far away from the BS have significantly lower SINRs than UEs close to the BS due to large-scale fading, shadowing, and other factors. In addition, the UL range, and hence the UL channel estimation accuracy, is limited by the transmitting power of UEs. Before the BS knows the channels of the UEs, it is unable to perform beamforming. This application presents embodiments that use Amplify and Forward Relays (AFRs) to extend the coverage of a MIMO BS and perform multi-user beamforming using channel estimates that include relays. Note that the AFR also can be called Capacity Projector (CaP) or repeater.
In [1], B. Rankov and A. Wittneben described relay-assisted wireless MIMO channels for single-user MIMO (SU-MIMO) where the destination antennas are equally spaced in a linear array, and the relays are limited to single antenna nodes. The relays use TDD, i.e., receiving a data packet in one time slot and transmitting it in another time slot. This reduces the spectral efficiency and requires synchronization of the relays.
In [2], C.-B. Chae et al. described MIMO relaying with linear processing for multiuser transmission in fixed relay networks that is also TDD two-hop communication, same as in [1].
Our embodiments use many relays that are spatially distributed and the relays do not store the message in one time slot and forward in a second time slot. Instead, our relays are full duplexing, receiving, amplifying, and re-transmitting simultaneously.
In [3], W. Xu and X. Dong described a limited feedback design for MIMO-relay assisted cellular networks with beamforming, in which each UE is required to feed back its quantized CSI to the relay, and the relay sends the quantized beamforming vectors to the BS. In addition, it is limited to the oversimplified case of (Number of antennas of the BS)=(Number of antennas of the relays)=(Number of antennas of the UEs).
In [4], M. Andersson and B. Goransson described a MIMO system with repeaters that has the following limitations: (1). It is for SU-MIMO, not MU-MIMO; (2). Another fundamental limitation is that it is beam steering not beamforming, that is, it is limited to steering antenna lobes to aim towards an repeater or UE, instead of real beamforming, which is constructive alignment of radio waves along multiple paths at the receiver; (3). Furthermore, it is limited to each beam from the BS being steered to one repeater, so the antennas in the BS are not used to perform multi-user beamforming to the repeaters, (4). It requires the repeaters antenna lobe to be aimed at the UE, and “at least one antenna radiation lobe of the second antenna of at least one repeater is electrically controllable” to be steered towards the UE, which means the channel between the UE and the repeaters must be estimated and beam steering must be performed on the repeaters. This means that the BS is not beamforming to multiple UEs holistically via the repeaters, i.e., it is not treating repeaters as part of the total channel from the BS to UEs in beamforming computations at the BS. In another word, the BS is not directly beamforming to the UEs. Instead, the steering of the beams is broken down to two stages, one stage is to steer a lobe from the BS to a repeater, and the second stage is to steer the lobe from the repeater to the UE. “The second antennas 24 of the repeaters 18, 19, 20, 21 then re-transmit the received respective information streams in such a way that the UE 12 now may have access to all four information streams.” “If there is more than one UE 12 in the cell 2, the controllable 25,26,27,28 lobe may switch between the UEs 12.”; (5). UE must have at least two antennas “the at least one repeater being arranged for communicating one of the MIMO communication streams to and/or from at least one UE (user equipment) the UE being equipped with at least two essentially uncorrelated antenna functions,”; and (6). The repeaters are not transparent to the BS and the BS requires the repeaters be arranged to communicate one of the BS communication streams to a UE. This means that the communication streams from the BS must be known a prior. In our invention, the communication streams from the BS are formed based on the total channel (TC) that includes one or more relays and one or more UEs, typically multiple relays and multiple UEs.
None of the prior art addressed the many technical challenges of a wireless network with MU-MIMO and a large number of relays widely distributed over a coverage area, including large number of antennas on the BS and relays, conditions and placement of full-duplex relays, efficient estimation of the TCs, and MU-MIMO beamforming using the TCs.
This invention presents significantly more advantageous solutions to dense networks using MU-MIMO and full duplex relays for both sub 6 GHz bands and for cm-wave and millimeter wave (mm-wave) bands. There is no prior art for efficiently extending the coverage of MIMO wireless nodes using relays as a method for densification of wireless networks.